With growing demands for higher speeds and larger capacities of data communications in recent networks, optical communication technologies which implement high-speed, large-capacity data communications are becoming important more and more. In the optical communication technologies, a coherent reception scheme, which is a reception scheme using the nature of interference of light, can obtain a higher reception sensitivity and has a high affinity for multilevel modulation such as QAM (Quadrature Amplitude Modulation), compared to reception schemes using modulation of modulating the intensity of light, such as OOK (On-Off Keying) and DPSK (Differential Phase Shift Keying) widely applied to conventional large-capacity optical communication systems.
However, the conventional coherent reception scheme cannot achieve stable reception owing to a frequency/phase offset of local oscillation (LO) light or polarization fluctuations generated in the respective polarization components of a multiplexed input optical signal.
To solve these problems, there is proposed a digital coherent reception scheme capable of stable reception by compensating for a frequency/phase offset and polarization fluctuations (patent literature 1). This scheme is implemented because a high-speed A/D (Analog to Digital) converter becomes available thanks to higher operation speeds and larger capacities of recent electronic devices, and digital signal processing for a signal obtained by digitally converting an optical signal becomes possible.
Patent literature 1 discloses an optical reception device which converts a received optical signal into a digital signal by A/D conversion processing, calculates an error from an optimal sampling timing in A/D conversion processing based on the digital signal, and corrects the sampling timing of A/D conversion processing.
FIG. 9 shows the arrangement of an optical reception device 40 using a digital coherent reception scheme as an example of the optical reception device proposed in patent literature 1.
As shown in FIG. 9, the optical reception device 40 demultiplexes a DP (Dual-Polarization: polarization multiplexed)-QPSK (Quadrature Phase Shift Keying) signal serving as an input signal into four channels (Ix, Qx, Iy, and Qy) via an optical element (90° hybrid). Photoelectric (O/E) converters convert components of the respective channels into analog signals. Then, A/D converters which perform sampling at timings synchronized with A/D identification clocks convert the converted analog signals into digital signals.
In the optical reception device 40, a DSP unit extracts an error from the optimal sampling timing in the A/D converter based on the A/D-converted digital signal, and corrects the sampling timing of the A/D converter. Accordingly, the optical reception device 40 can compensate for a frequency/phase offset and polarization fluctuations. By performing signal processing for the digital signal, the optical reception device 40 can also execute advanced waveform equalization processing such as wavelength dispersion compensation.
CMA (Constant Modulus Algorithm) is known as a conventional technique for implementing polarization fluctuation compensation among various compensations mentioned above (contents of a detailed principle of CMA are disclosed in, e.g., non-patent literature 1). For example, non-patent literature 2 discloses a receiver having a good transmission characteristic with little performance degradation by CMA at 50 ps in 1st-order polarization mode dispersion (PMD) in which the wavelength dispersion amount is 15,000 ps/nm at a 40-Gbps transmission rate.